1. Field
This application relates to communication networks and, more particularly, to a method and apparatus for providing integrated symmetric and asymmetric network capacity on an optical network.
2. Description of the Related Art
Data communication networks may include various computers, servers, hubs, switches, nodes, routers, proxies, and other devices coupled to and configured to pass data to one another. These devices will be referred to herein as “network elements”. Data is communicated through the data communication network by passing protocol data units, such as frames, packets, cells, or segments, between the network elements by utilizing one or more communication links.
A particular protocol data unit may be handled by multiple network elements and cross multiple communication links as it travels between its source and its destination over the network. To allow the network elements to work together, a large number of protocols have been developed. Some of the protocols operate at the physical layer to specify what the signals should look like, others operate at the link layer to handle end-to-end communication on a particular link, while still others operate at the network layer to define routes through the network for particular connections.
When there is a failure on a communication network, the traffic that is intended to pass through the failing part of the network will need to be sent a different way through the network. There are two common ways of causing this to occur—restoration and protection.
In restoration-based failure recovery, when a failure is detected on the network, the network elements will find one or more new paths through the network that are able to carry the affected traffic. Thus, in a restoration-based system, a new path will be set up after a failure has occurred. For example, a routing protocol operating at the network layer may be used to find a new route through the network that avoids the failure.
In a protection-based system, backup paths are set up in advance so that the backup paths are already determined should a failure occur. Upon occurrence of a failure, the traffic can simply be switched from the primary or working path to the alternate or protection path without waiting for a new path to be selected to carry the traffic. Traffic is protected on a working path if there is bandwidth on a protection path to carry the traffic in the event of a failure. At the network level, therefore, a protection path is a path that is reserved or specified as being configured to carry the working path traffic flows should there be a problem on the working path.
There are several different ways traffic may be protected. For example, the protection may be dedicated or shared. Dedicated protection refers to resources that are reserved solely to protect the connection or group of connections associated with the dedicated protection. Shared protection refers to having greater working traffic than protection capacity on the protected network. Also, more than one path may provide protection for a working path. Similarly, just because a path is designated as a protection path does not mean that it must be kept empty. For example, the protection path may carry less important traffic when not being used to carry traffic from the working path. Thus, there are many different ways in which protection may be established for a connection on a network.
The particular way in which network elements are interconnected is referred to herein as the network topology. For example, one common network topology is to interconnect a group of network elements into a ring formation with communication links interconnecting adjacent nodes in the ring. Synchronous Optical NETwork (SONET) and Synchronous Data Hierarchy (SDH) networks are commonly formed in a ring topology, although other technologies such as Resilient Packet Ring (RPR), Ethernet, Unidirectional Path Switched Ring (UPSR), and Bi-directional Line Switched Ring (BLSR) may also be used in a ring-topology network.
Several of the protocols that are used to implement ring-topology networks, particularly SONET and SDH, provide for the paths on the network to be protected so that fast protection switching may occur upon detection of a failure on a portion of the ring. Several protection schemes that may be used on a SONET/SDH network are commonly referred to as Bi-directional Line Switched Ring (BLSR), Head-end node Ring Switching (HRS), and MSPring, although other protection switching schemes may also exist and may be developed over time. References to SONET herein should be understood to include SDH and other optical ring technologies.
Initially, ring-topology networks were developed to support relatively symmetrical network traffic patterns. In particular, many of the original networks were designed to carry voice traffic which is relatively symmetric. For this reason, the rings that were deployed were designed to carry the same amount of working traffic in both directions around the ring. As new network services are developed, however, traffic patterns have changed such that subscribers are now generally consuming more bandwidth than they are generating. Additionally, as video content such as video on demand becomes more readily available to subscribers, the asymmetry of network traffic is expected to continue to grow.
Numerous companies have acknowledged that network traffic may be expected to become increasingly asymmetrical in parts of the network due to the emergence of video on demand and other video related services. So far, however, there hasn't been a clear plan as to how existing ring-topology networks may be adapted to accommodate these emerging asymmetrical network traffic patterns in an efficient manner. Rather, to accommodate the asymmetrical increase in traffic, network providers have been overlaying full rings on top of the original ring networks. Although these overlay rings enable the network to accommodate the asymmetrical increase in traffic in the downstream (toward the subscriber) direction, they do not do so in a cost-effective manner. Specifically, adding a full overlay ring with upstream as well as downstream capacity may result in the needless addition of upstream capacity to the network where the only requirement was that the downstream capacity be incremented.
To address this asymmetry, Cisco Systems™ has proposed a UniDirectional Link Routing (UDLR) system which allows unidirectional links to be deployed to handle asymmetric traffic, and to allow routing protocols to be run over unidirectional links. Unidirectional optical links may thus be deployed in a conventional network to increase the one way capacity of portions of the network. For example, such links could be deployed to enable a network to have large downstream and small upstream capabilities. Unfortunately, the proposed solution relies on a layer 3 (routing layer) to enable traffic to be passed over these unidirectional links. Requiring layer 3 processing introduces jitter and delay, as compared to an all optical solution. Since video signals are very sensitive to both jitter and delay, a layer 3 solution is less desirable than an all-optical solution. Additionally, the proposed UDLR system uses a head-end node based restoration scheme in which, upon detection of a failure, the routing protocol will generate a new route to circumvent the problem area of the network. For example, a common routing protocol such as Open Shortest Path First (OSPF) may be run across the UDLR-provisioned unidirectional links. Upon detection of a failure, these protocols may take up to 5 to 7 seconds to converge on a set of new routes for traffic on the network. Thus, while this solution does provide a way to accommodate asymmetrical traffic patterns, the convergence time required to recover from a fault on the network is too long to conform to the 50 ms recovery time specified by several of the telecommunication standards that govern provision of telecommunication services over communication networks.